The present invention relates to an integrated continuous process and the associated system for upstaging lower molecular weight liquid epoxy resins in one or more reactor chambers with one or more reaction zones in each chamber to produce a stream comprising a higher molecular weight product of resinous polyepoxides, hydroxy-terminated polyethers or phenolic terminated polyethers.
Epoxy resins have been known for many years. In combination with a suitable curing agent, such thermosetting resinous polyepoxides, have produced thermosetting polymers useful for many applications, providing a combination of superior toughness, flexibility, adhesion and electrical properties and chemical resistance in the finished products. The terms xe2x80x9cpolyepoxide,xe2x80x9d xe2x80x9cresinous polyepoxide,xe2x80x9d xe2x80x9cpolyethersxe2x80x9d and xe2x80x9cepoxy resinxe2x80x9d are used herein interchangeably.
The most common types of resinous polyepoxides are produced by reacting monomeric epoxy compounds, such as epichlorohydrin, with polyhydric (including dihydric) phenols, such as bisphenol-A (BPA), to give diglycidyl ethers. Depending primarily upon their molecular weights, resinous polyepoxides may vary from a viscous liquid to a high melting solid.
Higher molecular weight solid or semi-solid resinous polyepoxides, hydroxy- or phenolic-terminated polyethers are often made by a process known as xe2x80x9cupstaging,xe2x80x9d xe2x80x9cupgrading,xe2x80x9d xe2x80x9cfusionxe2x80x9d or xe2x80x9cadvancementxe2x80x9d. In such an upstaging or advancement process, a lower molecular weight liquid resinous polyepoxide is reacted with a polyhydric, most often dihydric, phenol in the presence of a catalyst until enough of the phenol is incorporated into the epoxy polymer chain, terminally and/or as a crosslinking agent, to increase the molecular weight of the upstaged epoxy resin product to the desired level. If the molecular weight is high enough, the product is in the solid form at room temperature. Other than using polyhydric phenols, it also is known that such an upstaging process may be carried out using carboxyl- or other hydroxyl-containing compounds or mixtures thereof.
The upstaging processes have in the past been carried out in both a batch process or a continuous process. See, for example, U.S. Pat. Nos. 3,547,881, 3,919,169 and 4,612,156. Typically in these known batch and continuous upstaging processes, the dihydric phenols and liquid polyepoxide are admixed or otherwise contacted with a catalyst at a relatively low temperature and then heated up to the reaction temperature and held at a desired reaction temperature and other conditions for a time sufficient to produce the resinous polyepoxide or hydroxy-terminated polyether of a higher molecular weight. Sometimes, the catalyst is added after the reactants have already been mixed and heated up to a higher temperature.
The cycle times are relatively long in typical batch upstaging processes. This cycle time includes charging of the raw materials, the upstaging reaction itself, discharging and solidification/packaging of the product. For example, a batch process involving bisphenol-A or tetrabromobisphenol A (TBBPA) and a liquid polyepoxide consisting essentially of the diglycidyl ether of bisphenol-A can take from about 8 to about 12 hours of cycle time to complete. It would be advantageous to shorten the cycle time needed in a batch process as well as the residence time in a continuous process to increase productivity and/or to reduce capital investment.
Furthermore, it is difficult to maintain the homogeneity of temperature in a large batch reaction vessel. Since the upstaging reaction is exothermic and the viscosity of the reaction mixture is usually quite high, there may be heat-transfer problems, localized hot spots and/or substantial temperature gradients inside the vessel. Unintended and adverse gelling, non-uniform upstaging, over- or under-crosslinking, localized side-reactions or byproduct formation also may take place as a result of non-uniform reaction conditions. All of these problems can lead to non-uniform inhomogeneous product compositions and/or product properties. For example, the product may exhibit broad molecular weight distributions, broad softening points or glass transition temperatures, inconsistent chemical compositions and others. In addition to such problems within a given batch, it is also not unusual to have significant batch-to-batch differences. These differences may cause additional problems in various applications due to varying molecular weight distributions and the associated changes in viscosity and other properties such as softening point or glass transition temperatures.
As mentioned earlier, certain continuous reaction systems have been proposed or disclosed to upstage epoxy resins using, for example, a long tubular (pipe) reactor (length/diameter=900) and a long tubular post-heat zone (I/d=1020) as disclosed in U.S. Pat. No. 3,919,169; or using a twin-screw extruder reactor as disclosed in U.S. Pat. No. 4,612,156. Long tubular or pipe reactors are generally known to subject to fouling problems due to heavies buildup and the upstaged products tend to show broad molecular weight distributions (so-called polydispersion or MW/Mn, the ratio of weight averaged molecular weight and number averaged molecular weight). The extruder reactor is an expensive piece of equipment for commercial scale productions. While these two patents might have disclosed certain general concepts with respect to continuous upstaging of epoxy resins, they disclose neither the use of a solvent in the system, nor the advantages of using non-tubular/non-extruder type reactors and/or the importance of particular modes of flow or flow directions of feeds.